A request by the Justice Department to add a citizenship question to the 2020 Census could shift the nation’s balance of political power from cities to more rural communities over the next decade and give Republicans a new advantage in drawing electoral boundaries.

Population numbers produced by the census are used in many ways, notably to draw political districts and distribute government funds across the country. Adding questions to the decennial survey is usually a controversial and difficult process because of the potential to affect both of those functions — either by suppressing census participation or by creating new ways to define populations.

All of it has prompted advocates for Hispanic communities to accuse the Justice Department of wanting to produce a less accurate count in 2020.

“I think the main motivation is to secure an undercount,” said Tom Saenz, president of the Mexican American Legal Defense and Education Fund. “Texas is a very red state. They know that is not going to be the case for very much longer.”

The citizenship question is a particularly fraught one because noncitizens, who may not vote, nonetheless are counted for the purposes of distributing federal funding, apportioning congressional seats and drawing district maps for state and local elections.

A majority of the nation’s undocumented immigrants live in just 20 metropolitan areas, according to a 2014 Pew Research Center study of Census Bureau data, numbering about 1 million in the New York and Los Angeles areas, 575,000 in Houston and 475,000 in Dallas. That makes urban leaders, mostly Democrats, alarmed by the possibility of the citizenship question — primarily because census data helps guide the distribution of more than $675 billion a year in federal funding.

“The Justice Department’s proposal to request citizenship status as part of the census is extremely damaging to the ability to secure an accurate count,” Houston Mayor Sylvester Turner said in a statement to The Washington Post.

The Census Bureau is expected to finalize the 2020 questionnaire by March 31. Officials have said they are evaluating the December request from the Justice Department and will make a decision in consultation with the White House Office of Management and Budget. Commerce Secretary Wilbur Ross, who oversees the Census Bureau, has not commented publicly on the idea.

‘Bad census data’

The census forms that are targeted to all households have not included questions about citizenship or country of birth since 1950. Instead, the government has acquired that data by surveying a sample of the total population.

Four former census directors warned in a 2015 legal filing that adding a question about citizenship for all households would undermine the accuracy of the count.

“The sum effect would be bad census data,” they wrote of the idea. “And any effort to correct for the data would be futile.”

In testing for the 2020 Census, immigrant communities have expressed a heightened reluctance to answer the coming questionnaire, even without any mention of a citizenship question.

Census researchers found that respondents in 2017 technology tests spontaneously brought up concerns about confidentiality and privacy at alarmingly high rates. In one test in the Washington area, four out of 15 people interviewed provided incomplete or inaccurate information because of those concerns, including several who mentioned fears that the data could be used by the government against immigrants.

“This frightened me, given how the situation is now,” said one Spanish-speaking respondent, when asked to explain why she did not report three of the four roomers living in her residence.

The federal government is barred by law from using raw, individualized census data for law enforcement. But confusion and suspicion have long been a barrier to getting an accurate count, and surveys have shown the problem growing in recent decades. During the 2000 cycle, census workers posted signs in immigrant communities that read, “NO INS. NO FBI. NO CIA. NO IRS,” in an effort to encourage participation.

The problem persisted. The Census Bureau found that in 2010, the government overcounted the non-Hispanic population by 0.8 percent, largely because people with multiple homes answered the survey multiple times. The same census undercounted the Hispanic population by 1.5 percent.

In some cases, the partisan effects could hurt Republicans.

Alabama, for instance, where an estimated 65,000 undocumented residents lived in 2014, is threatened with losing one of its seven congressional seats by a margin of no more than 10,000 total people after the 2020 count, said Michael Li, of the Brennan Center for Justice. In other words, decreased immigrant participation could be a deciding factor.

Because of the Voting Rights Act, the state’s lone majority-minority district is probably going to remain in place, meaning one of its majority-white Republican districts would have to go.

“Alabama is literally on the cusp,” he said.

Switching to eligible voters

Another political impact could occur years after the initial count, if conservative states with large immigrant populations attempt to use block-by-block citizenship numbers to change the way they draw state and local district lines.

Federal courts have long mandated that states use total population to decide federal apportionment and draw U.S. House districts. But those same courts have given more leeway to states when it comes to drawing lines for state legislative and local districts.

For years, conservative activists and partisan line drawers have discussed the possibility of switching from total population to eligible voters, or some combination of the two, when drawing state and local districts. The eligible voting population would not include noncitizens or children under the age of 18, who tend to be disproportionately minority.

“Some metric that equalizes citizen population and total population is fairer than creating a district in which only half of the people are able to vote next to a district where 100 percent of the people are able to vote,” said Edward Blum, the director of the Project on Fair Representation, who helped bring a Supreme Court case challenging the use of total population to create districts in Texas.

A switch to using eligible voters would effectively eliminate millions of Texans for the purposes of redistricting, leading to fewer urban districts and more rural ones. That would shift the balance of power away from heavily Hispanic areas, and likelymore heavily Democratic areas, where more noncitizens and children live.

“All of the districts with noncitizens in them and all of the districts with kids in them would have less representation,” said Andrew Beveridge, a demographer at Queens College in New York, who called the idea the “holy grail” for Republicans seeking to maintain a partisan advantage.

Using census data, Beveridge ran an experiment in 2016 to estimate the partisan effects of switching to eligible voters and concluded that Texas Republicans would pick up about seven state House seats and one Senate seat. If the same system were adopted in Florida, California or New York, Republicans were also likely to pick up seats, he found.

Any change in the redistricting process probably would be challenged at the Supreme Court. “There will be extensive litigation if it goes that route, unquestionably,” said Vanita Gupta, the president of the Leadership Conference on Civil and Human Rights.

In the December request to add the citizenship question, first reported by ProPublica, the Justice Department made no mention of partisan implications. Rather, the agency argued that its sole rationale was to provide better data for use in fulfilling Section 2 of the Voting Rights Act, which prohibits voting practices that discriminate against minorities.

“The Justice Department is committed to free and fair elections for all Americans and has sought reinstatement of the citizenship question on the census to fulfill that commitment,” spokesman Devin O’Malley said.

The rationale is different from the one contained in a leaked Jan. 23, 2017, draft executive order from President Trump focused on restructuring immigration policies, which an associate of the Justice Department, who spoke on the condition of anonymity, said was unrelated. That order called for the Census Bureau to add a citizenship question as part of a raft of immigration enforcement measures to “be more transparent with the American people.”

Conservative opponents of current immigration policies have long called for better census data on citizenship to clear the way for a shift away from using total population to draw political districts.

Republicans have repeatedly introduced bills mandating the collection of citizenship data — and its use in apportionment. Rep. Steve King (R-Iowa), an immigration hard-liner, has pushed for a constitutional amendment that would create congressional districts based on the number of U.S. citizens instead of total population.

The Justice Department proposal has been cheered as a step in the right direction by those who argue it is unfair that districts with different numbers of citizens get the same congressional representation.

“At a time when the idea of foreign actors manipulating our electoral process is being given considerable attention, counting illegal aliens in the census for apportionment purposes is a substantive threat to our electoral integrity,” Dale L. Wilcox, the executive director and general counsel for the Immigration Reform Law Institute, said in a statement. “To allow states with a large population of illegal aliens to be given political power at the expense of others is a perversion of the apportionment process.”

Michael Scherer is a national political reporter at The Washington Post. He was previously the Washington bureau chief for Time magazine, where he also served as the White House correspondent. Before joining Time, he was the Washington correspondent for Salon.com.